In recent years, the trend toward micro-scale pattern rule has been increasing with the trend toward large-scale integration and high-speed LSI circuits. The trend toward a shorter wavelength of the exposure light source lies behind it. For example, it became possible to mass-produce 64M-bit (processing dimension: 0.25 μm or less) DRAM (dynamic random-access memory) by the wavelength shortening from mercury lamp i-line (365 nm) to KrF excimer laser (248 nm). Nowadays, a lithography using ArF excimer laser (193 nm) has increasingly been examined, and a 65 nm node device has been examined by a combination with a high NA lens (NA≧0.9). Although the use of F2 laser (wavelength: 157 nm) had been named as a candidate for the production of the next 45 nm node devices, its application was postponed by many problems, such as cost increase of scanner, change of optical system, and low etching resistance of resist. As an alternative to F2 lithography, proposed was ArF immersion lithography. Now, its development is going on toward an early introduction.
As a resist suitable for such exposure wavelength, “chemically amplified resist material” attracts much attention. This material contains a radiosensitive acid generator (hereinafter referred to as “photoacid generator”), which generates an acid by radiation irradiation (hereinafter, referred to as “exposure”). Furthermore, it is a pattern-forming material that forms a pattern by making a difference in solubility between the exposed portion and the unexposed portion through a reaction using the acid generated by the exposure as a catalyst.
Examples of the photoacid generator used for such chemically amplified resist material include onium sulfonates, such as iodonium sulfonate and sulfonium sulfonate, sulfonic acid esters, N-imidosulfonate, N-oximesulfonate, o-nitrobenzylsulfonate, and tris(methane)sulfonate of pyrogallol.
Examples of the acid generated from the photoacid generator upon exposure include alkanesulfonic acids, arylsulfonic acids, and partially or entirely fluorinated arylsulfonic acids and alkanesulfonic acids.
Of these, acid generators that generate partially or entirely fluorinated alkanesulfonic acids have a sufficient acid strength in deprotection reactions of protective groups that are difficult in deprotection, and therefore many of them have been put into practical use. Their examples include triphenylsulfonium trifluoromethanesulfonate and triphenylsulfonium perfluoro-n-octane sulfonate. Although triphenylsulfonium trifluoromethanesulfonate generates a sufficiently strong acid to have a sufficiently high-resolution performance, it has a defect of high mask dependency as a photoresist due to low boiling point of the acid and due to long diffusion length of the acid. Triphenylsulfonium perfluoro-n-octanesulfonate has a sufficient acidity and is almost appropriate in terms of acid boiling point and diffusion length. Therefore, it attracts much attention in recent years. However, it should be noted that perfluorooctyl sulfonates might be hazardous to human health and the environment (see “Perfluorooctyl Sulfonates; Proposed Significant New Use Rule” dated Oct. 18, 2000 (Volume 65, Number: 202) from the U.S. Environmental Protection Agency).
Under such background, there have been the developments of acid generators that generate partially or entirely fluorinated alkanesulfonic acids and that have characteristics of having a sufficient acidity, being appropriate in terms of acid boiling point and diffusion length, and having less load on the environment. Thus, there have been the developments of alkoxycarbonylfluoroalkanesulfonic acid onium salts as acid generators, such as triphenylsulfonium methoxycarbonyldifluoromethanesulfonate (see Japanese Patent Application Publication No. 2004-117959), (4-methylphenyl)diphenylsulfonyl t-butoxycarbonyldifluoromethane-sulfonate (see Japanese Patent Application Publication No. 2002-214774 corresponding to U.S. Patent Application Publication Nos. 2002/0102491, 2005/0130060 and 2007/0003871), and triphenylsulfonium (adamantane-1-ylmethyl)oxycarbonyldifluoromethanesulfonate (see Japanese Patent Application Publication No. 2004-4561 corresponding to US Patent Application Publication No. 2003/0194639).
The following reaction scheme [1] is known to obtain an alkoxycarbonylfluoroalkanesulfonic acid onium salt [vii] (see Japanese Patent Application Publication No. 2004-117959 and U.S. Pat. No. 2,852,554). In this reaction scheme, 3,3,4,4-tetrafluoro-[1,2]oxathiethane 2,2-dioxide [iii] is synthesized by reacting tetrafluoroethylene [i] with sulfur trioxide [ii]. Then, the compound [v] is synthesized by a ring-opening reaction of the compound [iii] using an alcohol (ROH). Alternatively, the compound [v] is synthesized by a two-step reaction, that is, a ring-opening isomerization of the compound [iii] to obtain an oxyfluoride [iv] and then an esterification of the oxyfluoride [iv] into the compound [v] by an alcohol (ROH). Then, the compound [v] is turned into a sulfonate (a sodium sulfonate) [vi] by an alkali metal containing base, followed by a salt exchange with an onium salt such as sulfonium salt (Q+X− where Q+ is a monovalent onium cation and X− is usually a halogen ion), thereby obtaining the target acid generator, an alkoxycarbonylfluoroalkanesulfonic acid onium salt [vii].
